MPEG-4 Video Verification Model Version 7.0, Bristol, April 1997, MPEG-97/N 1642, ISO/IEC JTC/SC 29/WG 11 specifies an encoder and decoder for object-based coding of moving image sequences, where rectangular images of a fixed size are no longer coded and transmitted to the receiver within a video session (VS), but instead video objects (VO) of any size and shape are coded and transmitted. These video objects may then be further subdivided into different video object layers (VOL) to represent different resolution levels of a video object, for example. The image of a VO of a certain layer in the plane of the camera image at a certain time is the video object plane (VOP). Thus, the relationship between VO and VOP is equivalent to the relationship between image sequence and image in transmission of rectangular images of a fixed size.
The syntax for transmission of a VOP specifies first the signaling of the local time base of a VOP. This indicates the time with respect to previously transmitted VOPs at which the instantaneous VOP is to be displayed. FIG. 1 shows the syntax structure for elements VS, VO, VOL and the relevant parts for element VOP.
The parts of the VOP syntax shown here are relevant in this connection. The “modulo time base” element indicates the local time base of the VOP in increments of 1000 milliseconds, and the “VOP time increment” element also indicates the local time base in increments of one millisecond. The “VOP prediction type” element indicates which type of prediction is to be used for the VOP. There are four possibilities here: I-VOP, i.e. no prediction is used, P-VOP, i.e. the prediction is based on the preceding VOP, B-VOP, i.e. the prediction is based on the preceding and following VOPs, and S-VOP where the prediction is based on a SPRITE-VOP which is either transmitted once at the start of the video session or is derived from the reconstructed data during transmission.
In addition to transmission of the local time base of a VOP, the syntax specifies a possibility of signaling the “coded/not coded” state for a VOP. In the case of the “not coded” state for the VOP, no additional data is transmitted after the corresponding signaling elements, and if there is a new VOP, transmission thereof is begun. On the receiver end, a “not coded” VOP is not decoded further and is not displayed.
Here the “video object layer shape” element, which is specified in the area of the header info of the syntax of the respective VOL, indicates whether the VO is a rectangular VO (==0) or is a VO of any size and shape (≠0). Then for the case of a VO of any size and shape, the width of the rectangle surrounding the VOP is indicated with the help of the “VOP width” element. If this width is set to the value 0, this signals that the VOP has the “not coded” state. Then the transmission of the data of the instantaneous VOP is terminated and transmission of the next VOP is begun.